The present invention relates to a key switch structure adapted to a thin-gage electronic device such as an IC card having a key input portion.
There is, for example, shown a conventional type key switch structure in the Japanese laid-open patent application 60-91695. FIG. 3 shows a partially sectional view of the conventional key switch structure. In the drawing, a film-like movable contact 5 which can touch a fixed contact 4 provided on an interconnection board 3 is disposed on the inner surface of the key portion 2 of a front film 1. The contact 5 comprises a double phased adhesive film 6 and a carbon film 7 adhered to the lower surface of the film 6. The upper surface of the film 6 is adhesively secured with an adhesive material 8 onto the front film 1.
The contact 5 is disposed between a key aperture 10 formed in a front panel 9 and a key aperture 12 formed in a spacer film 11.
The above disclosed structure, however, has a shortcoming in that the front film 1 becomes gradually deformed by creep due to repeated depression of the key portion 2, decreasing the gap between the carbon film 7 and the fixed contact 4. This causes a depression power difference between each of the keys, so that the keys become difficult to use because of a sense of incongruity between the keys.
In the worst case, the carbon film 7 approaches the fixed contact 4 so that the carbon film 7 touches the fixed contact 4 without a key-in operation.